The present invention relates to a method for producing alkali carbonate from alkali hydroxide and carbon dioxide.
The production of alkali carbonate from lye by reaction with carbon dioxide or, more generally, carbon dioxide-containing gases has won much interest in the last two years. Due to the increasing need for chlorine in industry, increasing quantities of lye are becoming available through the electrolysis of alkali choride solutions. At least for a portion of this lye, there is no immediate demand.
The known methods for carbonating lyes have serious drawbacks.
A direct carbonating of the usual 50 weight-% sodium hydroxide lye leads, for example, to encrustations and crystal deposits at undesired spots in the apparatus during the conversion using carbon dioxide, and especially during the needed concentrating of the resulting soda solution, by evaporation, after removal of the precipitate. Moreover, the crystals of alkali carbonate often trap within themselves impurities in the form of sodium hydroxide or the small metal particles which are the result of wear. Even the process disclosed in German Published Application (Auslegeschrift) No. 1,138,748, which operates in two stages, cannot eliminate these disadvantages.
Evaporation of the excess water entering a process via the sodium lye may be carried out right in the carbonating stage, so that a special evaporating for concentrating of the above-mentioned soda solution can be eliminated. Since the released heat of neutralization is not large enough to supply the heat needed to evaporate the requisite amount of water, the necessary energy must be supplied either before the reaction, for example by strong heating of the high-percentage sodium lye according to German Laid-Open Application (Offenlegungsschrift) No. 1,567,921, or within the reactor such as in German Laid-Open Application (Offenlegungsschrift) No. 1,811,168 using steam coils. The solids are continuously removed from the hot suspension, while the filtrate, which mainly contains soda, is fed back to the reactor. The carbon dioxide or carbon dioxide-containing gas is bubbled from below through the reactor, which may for example be a carbonating tower or container equipped with a stirring device.
The above-described processes have the disadvantages that they work with a relatively large liquid volume and require a not insignificant expense for electrical energy. Additionally, should there be a failure somewhere in the system to cause the liquid to become still, then a critical situation immediately develops because crystals begin to settle in the carbonating container, pipe lines, pumps, etc., to cause a difficultly removable plugging-up of the system.
In the case of producing potassium carbonate, the practice has been such (See, for example, the description in Winnacker-Kuchler, CHEMISCHE TECHNOLOGIE, 3rd Edition (1970), Volume 1, page 222.) that the produced highly soluble potassium carbonate has been formed first dissolved in solution. The solution has had to be concentrated by evaporation. During cooling of the concentrated solution, hydrated potash of formula K.sub.2 CO.sub.3 . 11/2 H.sub.2 O crystallizes. This is filtered and dried at 120.degree.C to a commercial product containing 84 weight-% K.sub.2 CO.sub.3. A product substantially free of water of hydration and free of absorbed water is only obtained by drying at 250.degree. to 350.degree.C. The filtrate, containing about 50 weight-% K.sub.2 CO.sub.3, is mixed with fresh potassium lye and returned to the carbonating stage.
The previous practice for potassium carbonate production has not been satisfactory because, above all, the solution concentrating and crystallizing of carbonate require considerable expense for equipment. Because of the strong temperature dependence of the K.sub.2 CO.sub.3 -solubility, there have been difficulties for continuous operation. Crystal precipitation often has led to the plugging-up of pipe lines, valves, and other equipment.